1. Field of the Invention
The present invention relates to a high-pressure metal pipe having a connecting head and being formed of a thick-walled steel pipe of comparatively small diameter, as in the case of a high-pressure metal pipe used for a high-pressure fuel injection pipe approximately 4-20 mm in diameter and approximately 1-8 mm in wall thickness which is widely used as a fuel supply path in, for example, a diesel combustion engine, and to a method of forming such connecting head as well as a sleeve washer for such connecting head.
2. Description of the Prior Arts
This kind of high-pressure metal pipe having a connecting head as well as a method of forming the connecting head have heretofore been as follows. As shown in FIGS. 6 and 7 by way of example, a truncated conical connecting head 12 has a seat surface 13 made of an outside circumferential surface formed at the end of a thick-walled steel pipe 11, and as shown in FIG. 8 by way of example, a truncated conical connecting head 22 having an abacus-bead-like shape has a seat surface 23 made of an outside circumferential surface formed at the end of a thick-walled steel pipe 21. Each of the truncated conical connecting heads 12 and 22 is formed by buckling under axial pressure which is applied from the outside by a punch member, and at the same time a circumferential wall of each of the connecting heads 12 and 22 is expanded outwardly by buckling under such axial pressure, whereby an annular pocket 15-1 (FIG. 6) or 15-2 (FIG. 8) or an annular notch 15-3 (FIG. 7) which has an annular shape deep and large in cross section is formed around an inner circumferential surface of the connecting head 12 or 22. The high-pressure metal pipe is at present in use in the above-described state. Incidentally, reference numerals 14-1, 14-2 and 14-3 denote ring washers or sleeve washers which are respectively fitted to the backs of the connecting heads 12 and 22.
However, the high-pressure metal pipe having such a prior art connecting head as well as the method of forming the prior art connecting head have the following problems. One of the problems is that the deep and large annular pocket 15-1 or 15-2 or annular notch 15-3 (FIG. 8) which is formed in the inside of the connecting head allows a cavitation erosion to occur in the vicinity of the portion of the annular pocket 15-1 or 15-2 or the portion of the annular notch 15-3 owing to a high-pressure fluid during the use of the pipe, and the other problem is that there is a possibility that such a pocket or notch portion causes fatigue failure.
The present invention has been made in view of the above-described problems of the prior art, and aims to provide a high-pressure metal pipe having a connecting head capable of solving the risks of cavitation erosion and fatigue failure in the inside of the connecting head owing to the contour shape of an annular pocket or notch which is formed in the inside of the connecting head during the working thereof, the contour shape of the annular pocket or notch having a shallow depth and a gentle cross section, as well as to a method of forming such a connecting head.
To achieve the above object, according to an aspect of the invention, there is provided a high-pressure metal pipe which includes a connecting head constructed to have a sleeve washer having a preferably cylindrical pipe-shaped portion which covers an outer circumferential surface of the connecting head of reduced diameter, the connecting head of reduced diameter having an outside circumferential surface formed as a truncated cylindrical or truncated spherical seat surface corresponding to a mating seat, and being provided on a connecting end portion of a thick-walled steel pipe having a comparative small diameter, an annular concave groove which occurs in an inside portion of the connecting head during the formation thereof being shallow in depth and gentle in cross section. According to another aspect of the invention, there is provided a method of forming a connecting head for a high-pressure metal pipe which includes the steps of: fitting a sleeve washer having a cylindrical pipe-shaped portion onto a thick-walled steel pipe in the vicinity of a connecting end thereof, the thick-walled steel pipe being cut into a predetermined size in advance and having a comparatively small diameter; retaining the thick-walled steel pipe with a chuck with a head working section being left on a side which is closer to the connected end than is the sleeve washer; and working an outside circumferential surface of an end portion of the thick-walled steel pipe into a seat surface having a truncated conical shape or the truncated spherical shape corresponding to a mating seat, by a coaxial external pressure of a punch member having the shape of a connecting head, thereby forming the high-pressure metal pipe so that a cylindrical pipe-shaped portion the sleeve washer covers an outer circumferential surface of the connecting head which is of a reduced diameter and has in its inside an annular concave groove whose contour shape is shallow in depth and gentle.
The reduced-diameter connecting head which has the seat surface having the truncated conical shape or the truncated spherical shape has a maximum outer diameter exclusive of the sleeve washer, which is larger than the outer diameter of a straight portion of the thick-walled steel pipe by 10-45%, preferably 12.5-30.0%, more preferably 15-20%.
Moreover, the sleeve washer has a pipe portion which covers the outer circumferential surface of the connecting head.
In the present invention, the reduced-diameter connecting head which has the seat surface having the truncated conical shape or the truncated spherical shape is formed to have a maximum outer diameter exclusive of the sleeve washer, which is 10-45% larger than the outer diameter of a straight portion of the thick-walled steel pipe, for the following reason: in the case of less than 10%, the sleeve washer slips and cannot fasten a fastening nut, and during the fastening of the fastening nut, the connecting head is deformed or fractured by shearing, whereas in the case of greater than 45%, the annular concave groove assumes a contour shape having a deep depth and a cross section which abruptly changes, so that a cavitation erosion easily occurs and there is a likelihood that fatigue failure occurs.
Incidentally, the length of the connecting head is not limited to a particular length, and it is suitable to make the length of the connecting head larger in view of shearing force which acts during the fastening of the fastening nut, but an excessive length is not preferable, because a large pocket occurs.
In the present invention, since the connecting head can be reduced in diameter by the use of the sleeve washer which cover the outer circumferential surface of the connecting head, it is possible to reduce the stroke and the external axial pressure of a punch member during the formation of the connecting head, whereby it is possible to reduce the output of a forming apparatus and hence the size thereof and it is possible to change the contour shape of the annular pocket or notch to be formed in the inside of the connecting head into a contour shape having a shallow depth and a gentle cross section. Accordingly, it is possible to solve the risk of cavitation erosion due to fluid pressure in the inside of the connecting head, and moreover, it is possible to reduce the possibility that such a pocket or notch portion causes fatigue failure.